1. Field of the Invention
This invention relates generally to the field of data communications. More particularly, this invention relates to a method and apparatus for estimating phase roll in a data communication device such as an echo canceller or data modem.
2. Background of the Invention
When full duplex two wire telephone lines are used for data communication, echoes are often produced by impedance mismatches at the circuitry use to convert two wire transmission lines into four wire transmission lines. This circuitry generally takes the form of a hybrid transformer. If left uncancelled, such echoes can increase transmission errors to unacceptable levels.
Echo cancelers which are specialized to deal separately with near end echoes and far end echoes are described in U.S. Pat. No. 4,813,073. Representative echo cancelers are described in U.S. Pat. No. 4,232,400 to Yamamoto et al. and U.S. Pat. No. 3,903,377 to Sato.
One task of an echo canceller is to estimate characteristics of the network responsible for the echoes. One such characteristic which is often estimated is the phase roll produced by the network. Phase roll can be expressed as a frequency difference between signals transmitted and the frequency of the echo of the same signal when it returns to the transmitter. Estimation of this parameter is traditionally done by use of phase locking techniques such as those described by U.S. Pat. No. 4,072,830 to Gitlin et al., U.S. Pat. No. 4,813,073 to Ling and "A Passband Data-Driven Echo Canceller for Full Duplex Transmission on Two-Wire Circuits", S. B. Weinstein, IEEE Transactions on Communication, Vol. COM-25, No. 7, July 1977. While this and similar techniques have been effectively used to characterize phase roll, they are comparatively slow to converge to an accurate estimate of the phase roll.
In U.S. Pat. No. 4,588,187 to Billi et al., phase roll is calculated for use in cancelling voice echoes. This patent uses a pilot tone to train an echo canceller.
Normally, an echo canceller estimates the phase roll, initial value of phase and the tap values required for the particular channel The present invention provides a substantially more rapid convergence to a phase roll estimate by decoupling estimation of the phase roll frequency (or phase roll herein after) component from these other components normally estimated by an echo canceller. Once this is done, the other parameters may be estimated, for example, as described by Widrow et al. in "Adaptive Noise Cancelling: Principles and Applications", Proceedings of the IEEE, Vol. 63, Dec. 1975.